1. Field of the Invention
This invention relates to a sheathed heater, and more particularly to an improvement of a sheathed heater for use in a higher temperature atmosphere.
2. Description of the Prior Art
The conventional sheathed heater to be connected across a single-phase power source, as shown in FIG. 1, comprises a metal pipe 1, such as a stainless steel pipe, a core of porcelain insulator 2 made of magnesia or the like and arranged at the center of the metal pipe 1, a pair of lead pins 3a, 3b having base portions which are inserted into small holes formed in and passing through said core of porcelain insulator 2, an internal heater coil 4 wound around said core of porcelain insulator 2 and a thermal and electrical insulation material 5 of magnesia or the like filled in a space formed between the metal pipe 1 and the core of porcelain insulator 2, both ends of said internal heater coil 4 being electrically connected to said pair of lead pins 3a, 3b, respectively. Such a sheathed heater is reduced in diameter by a subsequent swaging process.
In such a conventional sheathed heater, however, if the length thereof is too long the sheathed heater tends to bend when it is subjected to said swaging process. Accordingly, it was contemplated to divide axially the core of porcelain insulator 2 into two or more core pieces, for example, into three core pieces 2a.about.2c, and a planar spacer 6 of magnesia or the like is inserted between adjacent core pieces, as shown in FIG. 2.
Further, in the conventional sheathed heater, the heater coil 4 has a limitation in length due to the diameter of the heater coil wire and the value of voltage applied thereto. Accordingly, said heater coil 4 is also divided into three heater coil segments 4a.about.4c, each wound around a core piece 2a.about.2c, and the voltage of a single-phase power source is applied by said lead pins 3a, 3b to the both ends of each heater coil segment 4a, 4b or 4c. Specifically, as shown in FIG. 2, one end A1, A2 or A3 of the heater coil segment 4a, 4b or 4c is connected to one of said lead pins 3a, 3b at one end of each of the core pieces 2a, 2b or 2c, and the other end B1, B2 or B3 of each heater coil segment 4a, 4b or 4c is connected to the other of said lead pins 3a, 3b at the other end of each of the core pieces 2a, 2b or 2c.
FIG. 3 shows a conventional sheathed heater to be connected across a three-phase power source.
It is publicly known that in the sheathed heater the insulation ability of the thermal and electrical insulation material 5 and the spacer 6 of magnesia or the like is lowered if the temperature is increased to more than about 800.degree. C. However, in the sheathed heater as shown in FIG. 2 or FIG. 3, a gap formed between the adjacent core pieces becomes a non-heated portion, so that the gap should be made small, such as about 2 to 3 mm in order to equalize the temperature distribution. If this is the case, a voltage, such as 100 V is applied between said gap, so that an abnormal current flows through said gap to heat this portion, thereby causing the service life of the sheathed heater to be shortened.